Bombesin/gastrin releasing peptide-like immunoreactivity existed in two molecular weight forms in rat hypothalamus, spinal cord and sensory ganglia. In the hypothalamus, this immunoreactivity was found in areas which did not contain substance P-immunoreactive cells. Met-enkephalin-arg-gly-leu-like immunoreactivity was widely distributed in rat brain and spinal cord. In the brain, this immunoreactivity was found in the same locations with other opioid peptides derived from preproenkephalin A. Motilin-like immunoreactivity was found in the arcuate nucleus of the hypothalamus. Using antisera against histamine, the brain histamine-containing neuronal system was revealed. The cell bodies were located in the basal hypothalamus around thepremammillary nuclei and in the caudal magnocellular nucleus.